The tropical crop sugarcane is of great economical interest, contributing to about two thirds of the world's raw sugar production (Pessoa Jr. et al., 2005). In some countries, part of the crop is destined to the production of ethanol, an important alternative energy source and a less polluting fuel. Due to its unique capacity of storing sucrose in the stems, sugarcane is an interesting model for studies on sugar synthesis, transport and accumulation. Sugarcane is a C4 grass capable of accumulating sucrose in its stems to levels exceeding 50% of its dry weight. Stem internodes mature progressively towards the base of the culm and there is a corresponding increase in sucrose concentration. Sucrose metabolism components and regulators are likely to be key players in determining sugarcane sucrose yield (Moore, 2005; Lunn and Furbank, 1999). Sugarcane is a complex polyploid grass with commercial varieties derived from conventional breeding. Recent yield data indicates that this technology may be reaching a limit in sugar productivity increases. It could be greatly advantageous to have genes associated with desirable traits targeted for directed improvement of varieties. Traditional breeding methods have been extensively employed in different countries along the past decades to develop varieties with increased sucrose yield, and resistant to plagues and diseases. Conventional varietal improvement is, however, limited by the narrow pool of suitable markers. In this sense, molecular genetics is seen as a promising tool to assist in the process of molecular marker identification. Knowledge on the genes that participate in sucrose content regulation may assist in the development of new varieties with increased productivity. This improvement is not only economically relevant, but has also a strong environmental appeal, considering it can lessen the need to expand cultivation areas and that ethanol is a source for renewable energy. Furthermore, a broader understanding of the highly specialized sugar production and accumulation mechanisms in sugarcane can bring new insights into sugar metabolism in other species.
Sugarcane is the common name given to the several species of the genus Saccharum, native to Asia, but cultivated for centuries in all five continents. It is a very efficient photosynthesizer making it one of the world's most important crop grasses. Sugarcane is perennial and has sturdy, jointed fibrous stalks 2-6 m tall, capable of storing large quantities of sucrose. Its cultivation requires warm and humid tropical or subtropical climate. Brazil, India and China are the largest producers. The major commercial cultivars are complex hybrids selected from crosses between S. officinarum, S. barberi, S. robustum, S. spontaneum and S. edule, as well as related genera that cross with Saccharum, such as Erianthus, Miscanthus, Narenga and Sclerostachya. 
S. spontaneum genotypes, found from Afghanistan to the South Pacific Islands, have the broadest geographical distribution in the genus Saccharum. Together with S. officinarum, it is the species most used in breeding programs aiming to improve vigor, fiber content, ratooning ability, environmental stress and disease resistance (Perez et al., 1997). The origin of S. spontaneum is not yet clear. It is believed that it might have originated from an introgression of Miscanthus, Erianthus and Sclerostachya (Roach and Daniels, 1987). S. officinarum genotypes have originated in New Guinea from S. robustum by natural and/or human selection. They produce thick stems and are capable of accumulating high levels of sucrose. They do not flower abundantly and are usually used as females in breeding programs (Perez et al., 1997).
The sequencing of 238 thousand sugarcane ESTs (Expressed Sequence Tags) by the Brazilian consortium SUCEST (Vettore et al., 2003) was a landmark for the sugarcane biotechnology field and also for the study of basic genetics and physiology of grasses. The ESTs were clustered and a total of 43 thousand SAS (Sugarcane Assembled Sequences) were identified and categorized (Vettore et al., 2003). Functional characterization of the transcripts can be viewed on the World Wide Web at sucest-fun.org.
This work describes the use of cDNA microarrays to identify genes differentially expressed in two sugarcane populations contrasting for sugar content. The methods used to identify differential expression, the construction of cDNA microarrays, hybridization conditions and data analysis have been previously described (Papini-Terzi et al., 2005). A total of 5154 genes had their expression profiled.
The plants analyzed in the present invention are derived from multiple crossings among S. officinarum and S. spontaneum genotypes and from commercial varieties that have been selected for sugar content for over 12-15 years. A useful strategy for target-gene identification has been denominated “genetical genomics”. First introduced by Jansen and Nap (2001), the method aims to apply large-scale analysis of gene expression to a segregating population. The use of cDNA microarrays to evaluate a sugarcane population that segregates for a certain trait may provide more insight into plant signaling and gene function than classical mutagenesis studies (Meyers et al., 2004). Although S. officinarum and S. spontaneum present a large genetic variability in nature, very few representatives participated in the generation of the modern commercial hybrids. Certainly, there are genes conferring favourable traits to be identified among them that can be explored in breeding programs. Likewise, the comparison of progenies from different commercial varieties carefully selected for sucrose enrichment is a strategy that can point to genes that have been selected for over the years by traditional breeding methods.